Log into the site editor and click the 'Insert' tab.
Under the 'Apps' heading in the left sidebar, choose 'Google Calendar'.
The widget will appear on the page. Double click on it to bring up the editing window.
Add the Gmail address the calendar is created under. NB: Make sure your calendar is public beforehand. Then click 'Apply'.
Click 'View my site' in the top right corner to see how the app displays on the page.